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Homework Help: Moment of inertia for this assembly

  1. Mar 10, 2017 #1
    1. The problem statement, all variables and given/known data
    A slender rod is 80.0 cm long and has mass 0.120 kg. A small 0.0200-kg sphere is welded to one end of the rod, and a small 0.0500-kg sphere is welded to the other end. The rod, pivoting about a stationary, frictionless axis at its center, is held horizontal and released from rest. What is the linear speed of the 0.0500-kg sphere as it passes through its lowest point?

    2. Relevant equations
    Ip = Icm + Md2
    Moment of inertia slender rod Ir = ML2/12
    -||- solid sphere Is = 2/5 MR2

    3. The attempt at a solution
    I'm right around the corner to solve this, I just need to find the sum of the systems moment of inertia. I have the center of mass of the system, I also know that I have to use the parallel axis theorem but I don't have the radius of the small spheres so I don't know how I am supposed to calculate their moment of inertia.

    Or should I be able to solve it without knowing the total moment of inertia?
  2. jcsd
  3. Mar 10, 2017 #2


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    Science Advisor
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    2017 Award

    You can consider the spheres as point masses on the basis of the term 'small' in the problem statement.
  4. Mar 10, 2017 #3
    Thanks! That was what I had to do!
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